This invention relates to hydraulically damped bushings comprising two rigid tubular bodies surrounding each other, at least a portion of said bodies being preferably generated by revolution, said bodies being concentric and coaxial at least when loaded, whereas an elastomeric member, which may be possibly a monolithic block, is mounted between said bodies, said elastomeric member being so shaped as to form with said bodies at least two sealed cavities which are diametrically located in opposite relationship along a direction D and communicate with each other by means of a narrow channel, the assembly consisting of said sealed cavities and channel being filled with a damping liquid.
Bushings of this kind are well known and intended to be mounted between two rigid members which may be coupled to said tubular bodies and able to be subjected, with the respect to each other, to oscillations oriented along the diametral direction D, the assembly being arranged in such a manner that, when at least certain oscillations are applied thereto, the liquid is alternately forced back from one sealed cavity to another and conversely through said narrow channel, thus providing for a given frequency of said oscillations (said frequency having a value F properly related to the size of the channel) a resonant effect which enables the transfer of said oscillations from one tubular body to the other to be damped.
The related rigid members could be, for instance, on the one hand a vehicle chassis, and, on the other hand, the internal combustion engine, the rear axle assembly or the front axle assembly of said vehicle.
The invention more particularly relates to, among the aforesaid bushings, those in which the portion of the elastomeric member, which defines at least one of the sealed cavities consists of a flexible tight diaphragm or bellows whose edges are attached in a tight manner onto the external tubular body (see U.S. Pat. No. 5,013,012).
The related cavity, so called "compensation cavity", is used for recovering with a large flexibility the variable volumes of liquid which are forced into said cavity from the other cavity, so called working cavity, and the related bellows does not substantially work for transmitting any load or other stresses from one of the rigid members to the other one, said transmission being achieved by a thick radial compression-withstanding portion integral to the elastomeric member and defining the working cavity.
Due to the relatively thin wall thereof, the bellows thus related is relatively flimsy and the fatigue behavior thereof may be limited when the bushing is subjected to cyclic oscillatory stresses of great amplitude, particularly when submitted to high extension strains.
That results in a reduced life period as regards the bushings provided with bellows of said type.
It is an object of this invention to obviate this disadvantage and thus to increase the life period of said bushings.
To this end, the bushings of the related type according to the invention are essentially characterized in that they comprise, within the cavity defined by the aforesaid bellows, a second tight bellows sealingly attached to the external tubular body, like the first one.
In the preferred embodiments, provision is made for any one of the following features:
the inner volume of the second bellows is vented to the open air through an aperture bored into the external tubular body; PA1 the bellows defining outwardly the compensation cavity has the shape of a strap which is transversely extending and bulged toward the bushing axis and the second bellows is similarly arranged within said cavity; PA1 the transverse cross-section of the strap forming the second bellows has the shape of a U or V especially having a flat bottom, the two legs and/or the bottom of which may be themselves corrugated.
Further the invention comprises, in addition to the aforesaid main features, certain peculiar features which could be preferably used simultaneously and which will appear more clearly on reading the following description.